Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752127Ab3JRI5Z (ORCPT ); Fri, 18 Oct 2013 04:57:25 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:35654 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751228Ab3JRI5X (ORCPT ); Fri, 18 Oct 2013 04:57:23 -0400 Date: Fri, 18 Oct 2013 09:56:38 +0100 From: Will Deacon To: "Jon Medhurst (Tixy)" Cc: Jiang Liu , Steven Rostedt , Catalin Marinas , Sandeepa Prabhu , Jiang Liu , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v4 2/7] arm64: introduce interfaces to hotpatch kernel and module code Message-ID: <20131018085638.GA2858@mudshark.cambridge.arm.com> References: <1381990781-27814-1-git-send-email-liuj97@gmail.com> <1381990781-27814-3-git-send-email-liuj97@gmail.com> <20131017113826.GJ18765@mudshark.cambridge.arm.com> <1382023441.19506.66.camel@linaro1.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1382023441.19506.66.camel@linaro1.home> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1970 Lines: 43 Hi Tixy, On Thu, Oct 17, 2013 at 04:24:01PM +0100, Jon Medhurst (Tixy) wrote: > On Thu, 2013-10-17 at 12:38 +0100, Will Deacon wrote: > > On Thu, Oct 17, 2013 at 07:19:35AM +0100, Jiang Liu wrote: > > > + /* > > > + * Execute __aarch64_insn_patch_text() on every online CPU, > > > + * which ensure serialization among all online CPUs. > > > + */ > > > + return stop_machine(aarch64_insn_patch_text_cb, &patch, NULL); > > > +} > > > > Whoa, whoa, whoa! The comment here is wrong -- we only run the patching on > > *one* CPU, which is the right thing to do. However, the arch/arm/ call to > > stop_machine in kprobes does actually run the patching code on *all* the > > online cores (including the cache flushing!). I think this is to work around > > cores without hardware cache maintenance broadcasting, but that could easily > > be called out specially (like we do in patch.c) and the flushing could be > > separated from the patching too. > [...] > > For code modifications done in 32bit ARM kprobes (and ftrace) I'm not > sure we ever actually resolved the possible cache flushing issues. If > there was specific reasons for flushing on all cores I can't remember > them, sorry. I have a suspicion that doing so was a case of sticking > with what the code was already doing, and flushing on all cores seemed > safest to guard against problems we hadn't thought about. [...] > Sorry, I don't think I've added much light on things here have I? I think you missed the bit I was confused about :) Flushing the cache on each core is necessary if cache_ops_need_broadcast, so I can understand why you'd have code to do that. The bit I don't understand is that you actually patch the instruction on each core too! Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/